The present invention relates to cable television (CATV) systems. More particularly, the present invention pertains to a method and apparatus for bypassing a digital on-screen display graphics insertion subsystem.
The wide spread use of analog video displays has created a need for displaying graphic images such as alphanumeric characters or other graphics along with analog video data. The graphics are typically laid over a video signal received from a separate remote source such as a broadcast television transmission, a video disk, a video tape or any other video source. Various arrangements are known for overlaying graphic images over a video signal received from such a separate remote video source.
U.S. Pat. No. 5,051,817 to Takano discloses a system for superimposing color characters on an input video signal. In this system, a first sync separator separates horizontal sync pulses from the input video signal. These horizontal sync pulses are used by a phase lock loop (PLL) circuit to generate a reference clock signal (P1) that is locked to the horizontal sync pulses of the input video signal. A second sync separator, a timing generator, a burst gate, and a second PLL circuit generate an oscillation output signal that is phase locked to a burst signal of the input video signal. The reference clock signal and the oscillation output signal are used to synchronize a generated character signal with the input video signal. A changeover signal generator generates changeover control signals to output only the input video signal, or the input video signal superimposed with color characters.
U.S. Pat. No. 5,541,666 to Zeidler et al. discloses a system for overlaying digital character signals on an analog source signal including a predetermined color sub-carrier which includes a sub-carrier phase locked loop, a digital character generating device, a digital video encoder and a switching device. The subscriber phase locked loop separately generates a color sub-carrier and a system clock signal which are locked to the color sub-carrier of the analog video source system. The digital character generating device detects horizontal and vertical timing of pixel information in the analog video source signal, and generates digital character signals that are to be overlaid in predetermined pixels of the analog video source signal. The digital video encoder is responsive to the color sub-carrier and system clock signals for generating a separate color sub-carrier which is locked to the color sub-carrier of the analog video source signal. The digital video encoder also converts the digital character signals from the digital character generating means into an analog video output signal that includes the color sub-carrier generated in the digital video encoder. The switching means directs the analog video output signal from the digital video encoder or the analog video source signal to an output of this system during times when the digital character is to be overlaid or not overlaid respectively on the analog video source signal.
A problem exists with these techniques in that insertion of digital information into an analog video source may only be required in certain time intervals. The insertion process inherently degrades the video signal. Signal degradation occurs both during time intervals when digital information is inserted and during time intervals when there is no digital information presented for insertion.